According to this invention, a process for producing fluorine containing polymer to obtain composite polymer electrolyte composition having excellent ion transport number, that is, ion transfer coefficient, for example, excellent transport number of lithium ion, is provided.A process for producing fluorine containing polymer comprising graft-polymerizing a molten salt monomer having a polymerizable functional group and a quaternary ammonium salt structure having a quaternary ammonium cation and anion, with a polymer having the following unit; —(CR1R2—CFX)— X means halogen atom except fluorine atom, R1 and R2 mean hydrogen or fluorine atom, each is same or different atom.